Remove nm_logging_syslog_closelog(). The reasons are:

  - closelog() is optional according to the manual.

  - we called nm_logging_syslog_closelog() at the end of the
    main() function. But we have destructors running afterwards,
    so we were closing the log before logging the last line.
    Apparently that had no bad consequences either, so why was
    closelog() even useful?
    Also, it's hard to determine when we log the last line and
    only closelog() afterwards.

  - closelog() does not revert what openlog() did, this is ugly.